Project Summary An increasing number of reports have demonstrated the presence of regulatory T cells (CD4+CD25+FoxP3+, Treg) in human neoplasms, including lung, cervical, and head and neck cancer. In the majority of these studies, these cells were shown to be suppressive in vitro. Experimental tumor models have shown that depletion of Tregs changes the immune response to tumors in vitro, and in selected cases, results in enhanced tumor immunity and rejection of end-stage tumors in vivo. Taken together, these studies clearly suggest that Tregs which selectively accumulate inside a tumor act to maintain a local cytokine environmentthat suppresses the effector function of tumor-infiltratingT cells. While active suppression by regulatory T cells plays an important role in the down-regulation of T cell responses to foreign and self-antigens in the peripheral immune system, very little is known about the role of Tregs in the context of central nervous system (CNS) biology. Our research group and our colleagues have been investigating the role of Tregs in malignant brain tumors. To date, we have been able to show that : (1) there is an increase of functional and immunosuppressive Tregs in the blood and tumor stroma of patients with glioma; (2) the degree of Treg infiltration depends on tumor grade and progression; (3) depletion of Tregs prolongs the survival of mice with experimental brain tumors; (4) the expression of heme- oxygenase-1 (HO-1), an enzyme with key anti-inflammatory, anti-apoptotic, and anti-proliferative actions, is linked to the induction of FoxP3 and (5) toll-like receptors (TLRs) are expressed on glioma as well as on glioma associated Treg (TLR4 and 9) and modulate the immune response to intracranial tumors in vivo. Our central hypothesis is that CD4+CD25+FoxP3+ T cells found in malignant glioma preferentially proliferate within the CNS and selectively inhibit the host immune response via HO-1 and TLR signaling, thereby directly contributing to the rapid progression of brain cancer. To examine this hypothesis, we now propose to pursue the following specific aims: (1) determine whether the increase in Tregs in brain tumors is due to migration or active proliferation within the CNS, (2) determine the requirement for HO-1 with respect to glioma infiltrating Tregs, and (3) dissect the immunosuppressive mechanisms and regulation of Tregs by TLR4 and 9 signaling to test our hypothesis that reversing the suppressive function of Tregs will enhance anti-tumor immunity in vivo.